Polarized quantum dot emission from photonic
crystal nanocavities studied under moderesonant excitation
R. Oulton1*, B.D. Jones1, S. Lam1, A.R.A. Chalcraft1, D. Szymanski1,2, D. O’Brien2, T.F.
Krauss2, D. Sanvitto3, A.M. Fox1, D.M. Whittaker1, M. Hopkinson4, M.S. Skolnick1,
1Dept.
of Physics and Astronomy, University of Sheffield, Hicks Building, Hounsfield Rd., Sheffield, S3 7RH, UK
2 School of Physics and Astronomy, The University of St. Andrews, KY16 9SS, UK
3
Universidad Autonoma, Depto. Fisica de Materiales C - IV, 612,C/ Francisco Tomás y Valiente nº 7, Cantoblanco,
28049 Madrid, SPAIN
4 EPSRC National Centre for III-V Technologies, Dept. of Electrical and Electronic Engineering, University of
Sheffield, Sheffield S1 3JD, UK
*Corresponding author: R.Oulton@Sheffield.ac.uk
Abstract: We study the linear polarization of the emission from single
quantum dots embedded in an “L3” defect nanocavity in a two-dimensional
photonic crystal. By using narrow linewidth optical excitation in resonance
with higher-order modes, we are able to achieve strong quantum dot
emission intensity whilst reducing the background from quantum dots in the
surrounding lattice. We find that all the dots observed emit very strongly
linearly polarized light of the same orientation as the closest mode, despite
the fact that these quantum dots may be spectrally detuned by several times
the mode linewidth. We discuss the coupling mechanisms which may
explain this behavior.
2007 Optical Society of America
OCIS codes: (000.0000) General.
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1. Introduction
Of the many proposals for the implementation of quantum information schemes, those
involving semiconductor quantum dots (QDs) embedded in a photonic structure have received
a great deal of attention [1], as they hold the promise of bridging the gap between atomic
cavity quantum electrodynamics and solid-state semiconductor devices. Particular attention is
being paid to increasing and controlling the interaction of the quantum dot with photons. By
embedding quantum dots into micropillars, microdisk resonators or photonic crystal
nanocavities, the light-matter interaction strength has been shown to increase to the point of
strong coupling [2-4]. One type of photonic cavity structure, the photonic crystal (PhC)
nanocavity, holds a great deal of promise, not only due to the very high quality factors and
ultra-low modal volumes achievable, but also because flexibility in cavity geometry is
inherent in the choice of cavity design. Many cavity designs have been demonstrated, along
with more complicated structures including coupled cavities and cavities coupled to 2-D
waveguides [5]. This is the point at which QD-cavity QED begins to extend beyond that
which is possible in atomic systems.
In this Article we present work on a particular type of PhC cavity, namely an “L3” defect
cavity [6], which has attracted a great deal of attention as the only PhC cavity in which strong
coupling with a quantum dot exciton has been observed [2,7]. In a previous publication, we
investigated the properties of the higher order modes that may be supported in these cavities
[8]. We demonstrate here that by exciting resonantly at the wavelength of these higher-order
modes, efficient optical excitation of both the fundamental cavity mode and individual
quantum dots inside the cavity may be achieved, whilst dramatically reducing background
emission from QDs outside of the cavity region. We also investigate the polarization of single
QD emission observed when exciting on-resonance with the higher-order modes. Our
previous work revealed that the emission of all of the L3 cavity modes is strongly polarized
[8]. Here we demonstrate that all of the single QD features we observe are strongly polarized
in the same direction as the closest mode, despite the fact that the quantum dots themselves
may be detuned by as many as sixteen times the mode linewidth. Such results not only
suggest that the excitation conditions result in only QDs inside the cavity region being
excited, but that a mechanism exists that allows coupling of the quantum dot transitions to a
mode even when far from resonance. These results are in agreement with a recent report
demonstrating off-resonant exciton-mode anticorrelation in a dot-cavity system where strong
coupling is observed on resonance [9].
2. Nanocavity Excitation Using Higher-Order Mode Excitation
The samples studied were fabricated from a wafer grown by molecular beam epitaxy
containing a 140nm thick GaAs membrane into which a single layer of InGaAs QDs at a
density of ~1010cm-2 was incorporated. Fig 1(a) shows the photoluminescence (PL) spectrum
of the unprocessed wafer (grey spectrum), revealing QD ensemble emission at 4K centred at
~910nm with a FWHM of ~40nm. The wafer was processed into hexagonal lattice PhC
structures of circular air holes using electron beam lithography and chemically-assisted ion
beam etching. A range of lattice constants 230 ≤ a ≤ 290nm, and fill-factors 0.25 ≤ r/a ≤ 0.33
was produced to provide a wide range of fundamental and higher order modes. “L3” defects
consisting of a line of three holes missing along the -K (“x”) direction of the lattice. The
two end holes were displaced along x by a factor s = 0.15a as described in Ref.[8].
As reported previously for similarly designed structures [8], several modes may be supported
in the cavity. Cavity emission is investigated using a standard micro-PL set-up as described
in Ref [8]. Fig 1(a) (red and blue spectra) shows the linearly polarization resolved PL
emission of a particular example, Cavity A, with lattice constant a = 275nm, using nonresonant illumination at 633nm. High intensity excitation is used such that the mode emission
dominates over any individual QD features.
The QD ensemble emission region covers five optical modes, with all showing a strong linear
polarization. Fig 1 (b) shows polar plots of the mode intensity vs linear polarization for
modes 1-3,5. The fundamental mode (M1) at 994nm has the highest Q-value of / ~5400.
Its polarization is  = 55%, defined as  = (Imax-Imin)/(Imax+Imin), where Imax and Imin are the
intensities at the maximum and minimum of the polar plot. The polarization axis is found to
lie at ~700 to the xdirection. Further lower-Q value modes are observed to shorter
wavelength, and are denoted M2 ( = 49% polarized along the y-direction, with Q ~ 1500),
M3 ( = 87% polarized along x with Q-value 1200), M4 (also polarized along x, but due to the
high density of QD features, the polarization magnitude and Q is difficult to determine) and
M5 ( = 88% polarized along y,Q-value ~1000). The polarization of these modes is discussed
in detail in Ref [8].
The design of this L3 cavity, with shifted end-holes, is optimized to maximize the Q-value of
the fundamental mode wavelength, by suppressing the coupling of light inside the cavity to
the outside environment. However, the higher-order modes are far leakier, as indicated by the
lower Q-values. As emission in the in-plane direction is expected to be strongly suppressed
due to the photonic crystal lattice [10], light emitted into the higher-order cavity mode couples
preferentially to the out-of-plane directions.
The fact that the higher-order modes are “leaky” in the vertical direction may be used to our
advantage. It has been shown in both L3 [11,12] and double-heterostructure type [13] cavities
that resonantly exciting at the wavelength of one of the higher-order modes using a narrow
(~20eV) linewidth laser results in strong light absorption, and subsequent intense emission
from lower-order modes, as well as suppression of background emission. We can explain this
observation by reference to the calculated cavity mode patterns. Fig 1 (c) shows the
calculated |E|2 intensity patterns for Modes 1-3 and 5, calculated in the centre of the
membrane for a structure with lattice constant a = 275nm and s = 0.15a. The calculated
spectral position, relative intensity and polarization of the modes were found to agree well
with the experimental data. Mode 4 is not shown – in fact calculations show that this is likely
to consist of two spectrally close, low Q modes. The polarization of both of these modes is
however predominantly along x.
Figure 1. (a) Photoluminescence spectra of Cavity A taken at high excitation power with
non-resonant excitation, detecting linear polarization at 00 (red) and 900 (blue) to the K photonic lattice axis. The grey spectrum shows the ensemble QD PL emission on an
unpatterned part of the wafer. (b) Polar PL intensity plots of modes M1- M3, M5. x (y)
corresponds to 00 (900) relative to the defect axis. Modes are labelled according to their
parity along the x (upper labels) and y axes (lower labels), as described in ref [8]. (c)
Calculated |E|2 mode patterns for modes 1-3 and 5 for Cavity A.
Whilst the intensity patterns vary between modes, it is clear that the electric field is
concentrated within the cavity region in all cases. Light impinging on the structure and
resonant with one of the higher-order modes will couple much more strongly to the areas in
the cavity where the electric field is non-zero. The light persists in the cavity longer, such that
the absorption probability increases. Carriers are subsequently excited and relax in energy to
the QD ground states, where they are observed in PL emission. Carriers are selectively
excited inside the cavity, and relaxation into the QD ground states is fast enough that carrier
diffusion throughout the device is highly unlikely to occur. This means that only QDs inside
the cavity region will be excited. This has a great advantage over optically exciting at
energies above the photonic band gap. In the latter case, absorption occurs with uniform
probability across the illuminated region. The spatial resolution is therefore governed by the
resolution of the microscope objective (in our case about 2m), which is much larger than the
cavity. In this case, QDs in the entire illuminated region are excited such that the desired
emission from a QD inside the cavity may be obscured by background QD emission in the
surrounding lattice.
Figure 2. (a) Multichannel photoluminescence excitation (PLE) spectra of Cavity A. The
excitation power is kept such that both the mode and several QD features are observed,
and the wavelength of the exciting laser stepped from 886nm (below the wavelength of
Mode 5) to 905nm (above the wavelength of Mode 5). A spectrum is taken at each step.
The spectra are shifted both vertically and horizontally for clarity. (b) Spectrum around
Mode 3 taken for the excitation wavelength which gives maximum intensity (indicated in
red in (a)). (c) Normalized intensity of Mode 3 (green squares), QD 1 (blue open
circles), QD 2 (black open diamonds) as a function of excitation wavelength (PLE
spectra). The spectral positions of these features are shown in (b). The PL of Mode 5
(red) taken from Fig 1(a) is also shown for comparison.
In Fig. 2 we demonstrate the operation of this technique in one of our devices, Cavity A. Here
the effect of exciting resonantly with a 20eV linewidth CW Ti:Sapphire laser into Mode 5
[see Fig 1(a)] is demonstrated by observing the PL emission around Mode 3. The laser is
tuned from below the Mode 5 wavelength at 886nm to above it at 905nm in 0.45nm steps,
with a CCD spectrum taken at each wavelength. The excitation polarization was matched to
that of Mode 5, and the excitation intensity adjusted such that both the background emission
from Mode 3 and single QD features were observed. The resulting multichannel
photoluminescence excitation (PLE) spectrum is show in Fig 2(a). Off-resonance with Mode
5, very little emission is observed, but as the laser is tuned into resonance, several PL features
appear which increase in intensity until the laser is exactly on resonance with Mode 5. Fig 2
(b) shows the PL spectrum taken when the laser is on resonance. A broad background arises
from the Mode 3 emission and peaks at the point marked M3. Superimposed onto the mode
emission are several sharp features that correspond to emission from single QDs. Careful
measurement of the intensity of features M3, QD1 and QD2 reveals that the intensity ratio of
on-resonant to off-resonant excitation is ~90:1 or greater. In fact no PL emission above that
of the background noise could be observed off-resonance at ex = 905nm.
To verify that the intensity of the emission does indeed correspond directly to resonant
absorption into Mode 5, Fig 2(c) shows a plot of the normalized intensity as a function of
excitation wavelength of feature M3 (green squares), as well as two single QD features (blue
and black open circles) labelled in Fig 2(b). Also shown is a normalized PL spectrum of
Mode 5 taken with non-resonant excitation. All three PLE spectra closely follow the intensity
profile of the PL spectrum of Mode 5, indicating that their absorption spectra are almost
entirely governed by the wavelength-dependent local light enhancement into Mode 5. This
implies that the absorption density of states is close to uniform over the measured range,
which might appear surprising, since for single QDs, one might expect to observe absorption
only when on resonance with a sharp excited state (as observed using similar techniques in
Ref [14]). We therefore speculate that a continuum of absorption states exists for these QDs,
as described in Refs [15,16]. If this is the case, all QDs with ground states below the
excitation energy would be excited as long as they have the correct spatial position. Thus this
resonant excitation technique would appear to be an efficient and simple way to excite
selectively all QDs inside the cavity which couple to the modes.
3. Determination of QD-Mode Coupling from Strongly Polarized Emission
Let us now consider the polarization of the single QD emission. In Fig 1, it was demonstrated
that each of the modes is strongly polarized due to the symmetries of the cavity and
underlying PhC lattice. In Ref [13] it was shown for a double heterostructure cavity that light
could only be resonantly coupled into the cavity mode if the excitation laser was co-polarized
to the higher-order mode. We tested this assertion for our case: again, the laser was
resonantly tuned to Mode 5, and the PL intensity from Mode 3 monitored. Fig 3 (a) shows
two spectra taken of Mode 3, with excitation polarization along y (co-polarized to Mode 5)
and along x (cross-polarized to Mode 5). In this case the excitation intensity was increased
such that only emission from the mode is observed. The detection polarization was copolarized to Mode 3 along x. Similarly to Ref [13], under cross-polarized excitation almost no
emission is observed at Mode 3, with the intensity ratio I(co):I(cross) of greater than 200:1.
We now turn our attention to the polarization of the emission around the Mode 3 region. As
before, the excitation was tuned to Mode 5 and the excitation polarization along y, copolarized to Mode 5. The excitation intensity was reduced to allow observation of single QD
features. Fig 3 (b) shows PL spectra for the region around Mode 3, with polarization
detection along x (co-polarized to Mode 3) and y (cross-polarized to Mode 3, but co-polarized
to Mode 5). As expected, the broad background emission from the mode, centered at 933nm,
is strongly polarized. However, several sharp features that correspond to single QD
transitions are also observable, both on-resonance and several nm away from Mode 3. Again,
all of these transitions are strongly polarized. The polarization of each QD line is indicated in
Fig 3 (c), where positive polarization indicates polarization at 0 0 along the horizontal axis. All
of the QD features have polarization  > 70%. These results are rather surprising, as strongly
polarized QD emission is typically only observed when carriers retain their spin orientation
from the excitation. This is not the case here, where the excitation is along y and the detection
polarization along x. Linearly polarized doublets may be observed in high resolution
experiments [17], but both orthogonal polarizations should be observed in the case where the
fine structure splitting is greater than the spectral resolution. One should therefore not expect
to observe strongly polarized QD emission in the orthogonal direction to the excitation
polarization.
One possible explanation is that the polarization of the QD emission is strongly influenced by
the presence of the optical cavity mode. Such effects have already been observed in QD
elliptical micropillar devices [18,19]. In this case a QD on resonance with a polarized mode
experiences Purcell enhancement and greatly enhanced light emission. Enhancement occurs,
however, only for the QD polarization that matches that of the mode: the orthogonal QD
polarization is not enhanced, and thus the QD polarization is strongly linearly co-polarized
with the mode.
Figure 3. (a) PL emission from Cavity A at Mode 3, taken for excitation with a CW TiSp
laser tuned to be resonant with the wavelength of Mode 5. Blue shows excitation
polarization co-linear with the polarization of Mode 5 (along x), red for opposite linear
polarization (along y). The excitation power was chosen to be strong enough that the
Mode 3 emission dominates over single QD features. The detection polarization was
chosen to match that of Mode 3 (along x). (b) PL spectrum around Mode 3 at lower
excitation intensity, showing both QD and mode features. The excitation polarization
was kept to vertical, with the detection polarization vertical (blue) or horizontal (red). (c)
Linear polarization, defined as  = [I(00)-I(900)]/[I(00)+I(900)] of single QD features from
(b). The dashed line indicates the polarization of Mode 3 at high power.
Fig 3 (b) shows several QD lines over the spectral range 931-935 nm superimposed on the
Mode 3 emission. The lines are all strongly co-polarized ( > 90%) with Mode 3. Clearly,
QD polarization is enhanced when on resonance with the mode. However, the behavior of
QDs detuned from Mode 3 is also surprising: in the spectral region 938-941nm four QD lines
are observed that are also strongly co-polarized ( = 75-90%) to Mode 3. Given that these
features are at least one mode linewidth away from Mode 3, one would expect a greatly
reduced coupling to the mode, and almost no enhancement of the QD polarization. We
contrast these results in particular to those found in elliptical micropillars [19], where it was
found that a QD feature temperature-detuned to approximately one linewidth away from the
mode showed  ~ 0. The elliptical micropillar, however, has two orthogonal modes separated
from each other by just a few nm, such that the system is not strictly comparable to the PhC.
In order to investigate the effect of a detuned mode on the QD polarization in more detail, QD
emission close to the fundamental mode was chosen for study, as this mode has a greater Qvalue. The spectral detuning relative to the width of the mode is therefore very large, even for
QD lines only a few nm away. As the fundamental mode in Cavity A lies at 994nm, the
probability of finding a QD inside the cavity at this long wavelength is very small. Therefore,
a second Cavity B, with lattice constant a = 260nm, was studied. In this cavity the
fundamental mode occurs at 948nm, which is closer to the QD ensemble distribution.
Resonant excitation was performed into a higher-order mode occurring at 895nm, with
excitation polarization at 00. Fig 4 (a) shows PL spectra around the fundamental mode, taken
at 900 (y) and 00 (x). Again, the excitation intensity was adjusted such that both QD and mode
emission were observed. The broad feature observed at 948.5nm corresponds to the
fundamental mode, with a Q-value of ~5300 and linewidth of  = 0.18nm. The QDs lines
that are just a few nm away are detuned by several linewidths. The QDs far from resonance
from the mode have weak emission, while those on resonance have stronger emission. As in
Fig 3(c), Fig 4(b) shows the polarization of both the mode and the QD lines, with positive
polarization this time indicating polarization along y, co-polarized to the fundamental mode.
While the fundamental mode is not as strongly polarized as the higher order modes (fund =
80%), one observes a strong polarization of all the QDs, 58 % <  < 100 %, over a spectra
range 946-950nm. Let us take for example the weak line at 945.5nm (QD1), which has a
polarization of  = 76%. It would appear that the polarization of this QD is strongly
influenced by the fundamental mode, despite the fact that it is detuned from the mode by 3nm,
i.e. over 16 .
Figure 4. (a) Photoluminescence spectra of Cavity B. The broad feature observed
corresponds to the fundamental mode, and the sharp features to single QD lines. The
PL is detected co-polarized (y - blue) and cross-polarized (x - red) to the mode. (b)
Polarization of the mode and single QD lines observed over this spectral range. Positive
polarization is along the y-direction. (c) PL spectra taken for increasing temperature,
showing the mode and QD2 on the long wavelength side only. (d) Polarization of mode
(blue) and QD (red) as a function of temperature.
To monitor the polarization of a particular QD as a function of the detuning from the mode,
the temperature was increased, and the QD line close to resonance at 948.9nm at 4K (QD2)
shifted away from resonance. Fig 4 (c) shows a series of PL spectra covering the fundamental
mode and the QD for a temperature range 4-40K. The mode and QD polarization is plotted in
Fig 4 (d). Despite the fact that the QD is detuned from the mode up to ~1.5nm (~8away)
no change in the polarization is observed.
The strong polarization of the QD emission lines, even when detuned by up to 16 linewidths,
implies that the QD emission couples to the mode even when far from resonance. Purcell
enhancement of one particular mode polarization would lead to an increased spontaneous
emission rate of that particular polarization only. As the Purcell enhancement also generally
leads to an increase in emission intensity, the emission is consequently strongly polarized.
For such large detuning one might expect that Purcell enhancements of emission rate would
be negligibly small. For a detuning of 16 the Purcell enhancement should be ~10-3 times
the value on resonance. One should consider, however, the fact that emission into leaky
modes may be suppressed in the in-plane direction, such that the spontaneous emission rate in
the 2D photonic crystal is reduced by an order of magnitude [10]. This means that Purcell
enhancement would remain dominant, even for stronger detuning. Nevertheless, a strong
dependence on the detuning of the QD relative to the mode should be observed.
An alternative explanation is that only emission into the cavity mode is detected even at large
detunings, since emission into leaky modes may not be coupled efficiently to the detection
optics. As a result, strong polarization of the QD emission will be found, determined by the
polarization of the closest cavity mode and independent of the detuning, since the cavity mode
itself will be strongly polarized for all detunings from its peak.
While the exact mechanism responsible for coupling the QD to the mode is not understood,
these results imply that the mode has a significant influence on the QD emission, even when
far from resonance. These results are in line with results in ref [9], which show that in a
system where strong coupling was observed on-resonance, the mode and QD are correlated on
the single-quanta level, despite being strongly detuned. Our results suggest, therefore, that
one cannot consider a QD in a photonic crystal as an ideal two-level system, and that a
coupling phenomenon occurs in photonic crystals, that is not unique to the strong coupling
regime.
4. Conclusions
In conclusion, we have used mode-resonant excitation for selective excitation of individual
quantum dots inside a photonic crystal nanocavity. By resonantly exciting into higher-order
photonic modes, carriers are absorbed in QDs in the cavity region only. The technique results
in efficient excitation of the QDs, and suppression of background PL from the surrounding 2D
PhC lattice. We measured the linear polarization of the individual QDs as a function of
spectral detuning from higher-order and fundamental modes, and found that the QD lines
show the same very strong linear polarization as the modes themselves. This effect persists
even when the QD and the mode are spectrally detuned by as much as 16 mode linewidths.
While the Purcell enhancement of one QD polarization with respect to the other may explain
our observations, one would have to assume a very strong Purcell enhancement for all the
QDs observed, and strong spontaneous emission suppression in the in-plane direction. This
explanation appears unlikely. We therefore suggest that our QD is not an ideal two-level
system, and that an additional coupling mechanism exists.